Superconductivity is a phenomenon occurring at very low temperatures in some materials, in which the electrons responsible for conduction undergo a collective transition to an ordered state, of which superconductivity is a characteristic. This ordered state exhibits several unique and remarkable properties: disappearance of resistance to the flow of electric current, appearance of a large diamagnetism and other unusual magnetic effects, substantial alteration of many thermal properties, and the occurrence of quantum effects otherwise observable only at the atomic and subatomic level. The temperature below which a material begins to exhibit superconductivity is called the transition temperature or "critical temperature," usually designated T.sub.c. Below the critical temperature, electrical resistance of low-temperature superconductors drops sharply to levels at least 10.sup.12 times less than at normal temperatures. In high-temperature superconductors in the microwave and millimeter wave regions, the resistance drops sharply to levels on the order of 10.sup.3 to 10.sup.4 times less than at normal temperatures.
Other phenomena beside the disappearance of electrical resistance are displayed by superconductors. One of these is the Meissner-Ochsenfeld effect, in which an applied magnetic field is excluded from the interior of the superconductor. As long as the magnetic flux in a superconductor is low, the superconductor will remain completely superconducting in an applied magnetic field. If the magnetic field becomes too large, however, the superconductor will become partially or totally normal. That is, when the magnetic field exceeds a "critical field," designated H.sub.c1, the superconductor reverts to the normal state and its resistance to electric current rises sharply.
Related to the Meissner-Ochsenfeld effect is the phenomenon of penetration depth. The way in which a superconductor excludes from its interior an applied magnetic field smaller than the critical field H.sub.c1 is by establishing a persistent supercurrent on its surface and inside the material to the penetration depth which exactly cancels the applied field inside the superconductor. This current flows in a very thin layer of thickness .lambda., which is called the penetration depth. The external magnetic field also penetrates the superconductor within the penetration depth. Lambda depends on the material and on the temperature, and is typically very small, on the order of 1500 to 5000 Angstroms.
The existence of the critical field leads to another property of superconductors which is of importance. A supercurrent flowing in a superconductor will itself create a magnetic field, and this field will drive the superconductor normal at some critical value of the current, called the effective critical current. When the current in the superconductor exceeds the critical current density, the superconductor becomes normal and its resistance increases sharply.
The present invention is applicable to superconducting epitaxial thin films whose film thickness can be controlled up to at least a few depths of penetration of the superconducting material and can handle high current densities at temperatures below critical temperatures. Such thin films are usually deposited on substrates whose maximum size for uniform films is limited by the state of the art of film deposition and size of substrates or by cost, or both.
The invention pertains to optimizing the design of flat spiral inductances and, where desirable, of associated thin film capacitances utilizing the limited surface area available with superconducting thin films. Such films can handle currents up to 40 million amperes per square centimeter (A/cm.sup.2). For a film thickness on the order of 2000 Angstroms, the line, width needed for a superconductor to carry one Ampere would be approximately 12.5 microns (12.5 .times.10.sup.-6 meters). This enables a large number of conductors to be placed in a small area.
By controlling line width and depth, flat spiral inductances can be optimized for maximum current handling or minimum losses. By controlling, i.e., reducing, the spacing between turns of the spiral, mutual coupling between turns, and therefore inductance, can be increased, albeit at the expense of lowering the spiral self-resonant frequency. However, closer spacing between turns increases field strength between turns, causes parasitic capacitance between turns and can cause voltage breakdown.
Another potential problem is that, whenever normal (i.e., non-superconducting) current leads are attached to a superconductor, especially a narrow line thin film superconductor, power delivered to the contact can cause localized heating of the superconductor and raise its temperature above the critical temperature. Connections between separate inductors and separate capacitors can also cause this problem.